1. Field of the Invention
The present invention relates to a cable with recyclable covering. In particular, the invention relates to a cable for transporting or distributing medium or high voltage electric energy, wherein an extruded covering layer based on a thermoplastic polymer material is admixture with a dielectric liquid with superior mechanical and electrical properties is present, enabling, in particular, the use of high operating temperatures and the transportation of high power energy.
2. Description of the Related Art
The requirement for products of high environmental compatibility, composed of materials which, in addition to not being harmful to the environment during production or utilization, can be easily recycled at the end of their life, if now fully accepted in the field of electrical and telecommunications cables.
However the use of materials compatible with the environment is conditioned by the need to limit costs while, for the more common uses, providing a performance equal to or better than that of conventional materials.
In the case of cables for transporting medium and high voltage energy, the various coverings surrounding the conductor commonly consist of polyolefin-based crosslinked polymer, in particular crosslinked polyethylene (XLPE), or elastomeric ethylene/propylene (EPR) or ethylene/propylene/diene (EPDM) copolymers, also crosslinked. The crosslinking, effected after the step of extrusion of the polymeric material onto the conductor, gives the material satisfactory performance even under hot conditions during continuous use and with current overload.
It is well known however that crosslinked materials cannot be recycled, so that manufacturing wastes and the covering material of cables which have reached the end of their life can be disposed of only by incineration.
Electric cables are also known having their insulation consisting of a multi-layer wrapping of a paper or paper/polypropylene laminate impregnated with a large quantity of a dielectric liquid (commonly known as mass impregnated cables or also oil-filled cables). By completely filling the spaces present in the multi-layer wrapping, the dielectric liquid prevents partial discharges arising with consequent perforation of the electrical insulation. As dielectric liquids products are commonly used such as mineral oils, polybutenes, alkylbenzenes and the like (see for example U.S. Pat. No. 4,543,207, U.S. Pat. No. 4,621,302, EP-a-0987718, WO 98/32137).
It is however well known that mass impregnated cables have numerous drawbacks compared with extruded insulation cables, so that their use is currently restricted to specific fields of application, in particular to the construction of high and very high voltage direct current transmission lines, both for terrestrial and in particular for underwater installations. In this respect, the production of mass impregnated cables is particularly complex and costly, both for the high cost of the laminates and for the difficulties encountered during the steps of wrapping the laminate and then of impregnating it with the dielectric liquid. In particular, the dielectric liquid used must have low viscosity under cold conditions to allow rapid and uniform impregnation, while at the same time it must have a low tendency to migrate during installation and operation of the cable to prevent liquid loss from the cable ends or following breakage. In addition, mass impregnated cables cannot be recycled and their use is limited to an operating temperature of less than 90° C.
Within non-crosslinked polymeric materials, it is known to use high density polyethylene (HDPE) for covering high voltage cables. HDPE has however the drawback of a lower temperature resistance than XLPE, both to current overload and during operation.
Thermoplastic low density polyethylene (LDPE) insulating coverings are also used in medium and high voltage cables: again in this case; these coverings are limited by too low an operating temperature (about 70° C.).
WO 99/13477 describes an insulating material consisting of a thermoplastic polymer forming a continuous phase which incorporates a liquid or easily meltable dielectric forming a mobile interpenetrating phase within the solid polymer structure. The weight ratio of thermoplastic polymer to dielectric is between 95:5 and 25:75. The insulating material can be produced by mixing the two components while hot either batchwise or continuously (for example by means of an extruder). The resultant mixture is then granulated and used as insulating material for producing a high voltage electric cable by extrusion onto a conductor. The material can be used either in thermoplastic or crosslinked form. As thermoplastic polymers are indicated: polyolefins, polyacetates, cellulose polymers, polyesters, polyketones, polyacrylates, polyamides and polyamines. The use of polymers of low crystallinity is particularly suggested. The dielectric is preferably a synthetic or mineral oil of low or high viscosity, in particular a polyisobutene, naphthene, polyaromatic, α-olefin or silicone oil.
The Applicant considers as still unsolved the technical problem of producing an electric cable with a covering made from a thermoplastic polymer material having mechanical and electric properties comparable to those of cables with an insulating covering of crosslinked material. In particular, the Applicant has considered the problem of producing a cable with a non-crosslinked insulating covering having good flexibility and high mechanical strength under both hot and cold conditions, while at the same time possessing high dielectric strength, without using products potentially polluting during the life cycle of the cable, i.e. from its production to its disposal.
In view of said problem, the Applicant considers that the addition of dielectric liquids to polymer materials as proposed in the cited WO 99/13477 gives totally unsatisfactory results. In this respect, the Applicant maintains that adding a dielectric liquid to an insulating material should both determine a significant increase in its electrical properties (in particular its dielectric strength), without changing the material characteristics (thermomechanical properties, manageability) and without resulting in exudation of the dielectric liquid. In particular, the resultant cable should give substantially constant performance with time and hence high reliability, even at high operating temperatures (at least 90° C. and beyond).